Method for improving steel for carbonated beverage containers

ABSTRACT

A METHOD OF MANUFACTURING TINPLATE OR OTHER COATED SHEET STEELS FOR CARBONATED BEVERAGE CONTAINERS WHEREIN THE SHEET STEEL, FOLLOWING THE CONVENTIONAL COLD ROLLING PROCEDURE, IS IMMERSED IN AN AQUEOUS SOLUTION OF FERROUS SULFATE AND SULFURIC ACID, DRIED AND THEN ANNEALED IN A REDUCING ATMOSPHERE CONTAINING HYDROGEN.

United States Patent Ofice 3,707,408 Patented Dec. 26, 1972 ABSTRACT OF THE DISCLOSURE A method of manufacturing tinplate or other coated sheet steels for carbonated beverage containers wherein the sheet steel, following the conventional cold rolling procedure, is immersed in an aqueous solution of ferrous sulfate and sulfuric acid, dried and then annealed in a reducing atmosphere containing hydrogen.

BACKGROUND OF THE INVENTION Tinplate and other coated sheet steels used in the fabrication of food containers or cans are usually manufactured by hot rolling a conventional low-carbon steel to about 0.08 inch. This product, called hot band, is subsequently cleaned of all surface scale and cold rolled to less than about 0.020 inch, more particularly, to about 0.017 inch for beverage can end stock and to about 0.010 inch for beverage can body stock. After the initial cold rolling the cold rolled strip is annealed in a non-oxidizing atmosphere at about 1200 F. or more to relieve the cold rolling stresses. For the respective gages mentioned a second cold reduction to about 0.012 inch and 0.004 inch is employed after annealing. The strip is then coated with a corrosion resistant material such as tin, chromium or organic coating.

While these are the steps presently employed the hot band may be reduced to the finsh gage (i.e., about 0.012 inch or about 0.004 inch), annealed as above, and then temper rolled to achieve shape and flatness. The strip is ,then coated as above.

Even though the steel strip is tinned or chromium plated, and the interior surfaces of the manufactured containers are usually further coated with a protective resin, the containers life may be severely limited in the presence of some corrosive food products. For example, many carbonated beverages packed in tinplate or tin-free steel cans are known to quickly attack the container material exposed through fiaws in the resin coating. Therefore, the resistance of the base steel to direct attack by the carbonated beverage, is considered to be the main factor controlling the rate of perforation at such areas of exposed metal throughout the major part of the can life.

It is generally well known in the industry that steels containing a small amount of sulfur, approximately 0.03%, have a substantially improved resistance to the corrosive effects of carbonated beverages compared with steels containing lower amounts of sulfur. Therefore, in the manufacture of steel container products for use by the carbonated beverage industry, it has been the practice to use a resulfurized steel for the base metal, that is, a steel to which sulfur has been added to achieve a concentration of about 0.03%. It follows therefore that even though a carbonated beverage container may have flaws in the resin coating, thereby permitting the carbonated beverage to directly contact the base metal, the base metal being a resulfurized steel, will resist the corrosive action thereby greatly enhancing the life of the container.

Resulfurization of the steel is usually accomplished by adding elemental sulfur to the molten metal in the ladle prior to casting and rolling, with the usual aim being about 0.03% sulfur. This method of resulfurizing steel, i.e., direct sulfur additions, does however present numerous problems to the mill operator. For example, the de sired sulfur concentration in hot rolled steel may be sufiicient to physically segregate, resulting in non-uniform corrosion resistance in the final product. In the newer, commercially, continuous cast steels, sulfur levels of about 0.025% or higher often result in substantial surface defects in the cast slabs which require excessive scarfing prior to hot rolling.

SUMMARY OF THE INVENTION This invention is predicated upon our conception and development of a new and improved method for manufacturing a sulfur-containing sheet steel product for use in making carbonated beverage containers which overcomes the above noted disadvantages associated with the use of conventional resulfurized steel. By this process a low-sulfur steel, i.e. nonresulfurized, is hot and cold rolled to the required gage, followed by a unique surface sulfurizing treatment to produce a sheet having excellent resistance to the corrosive effects of carbonated beverages. Because the steel does not contain an appreciable amount of sulfur during the hot and primary cold rolling operations, the usual problems as noted above are overcome to virtually eliminate the rejected strip. The resulting cost efficiencies are obvious.

It is therefore an object of this invention to provide an improved process for producing sulfur containing sheet steels for use in the fabrication of carbonated beverage containers.

It is another object of this invention to provide a method for annealing cold rolled sheet products that will impart a sufiicient surface sulfur concentration to the steel to render the steel more resistant to the corrosive effects of carbonated beverages.

It is a further object of this invention to improve the conventional methods of manufacturing sheet steel for carbonated beverage containers by providing a surface sulfurizing treatment thereby permitting the utilization of more conventional nonresulfurized steels through the rolling processes.

It is yet a further object of this invention to provide a new and improved sheet steel for use in the fabrication of carbonated beverage containers having a low core sulfor concentration, i.e., less than 0.025% and a surface sulfur enrichment corresponding to a sulfur weight gain of about 0.05 milligram per square centimeter (mg/cm?) or more.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT Since the crux of this invention resides in the surface sulfurizing treatment following primary cold reduction of the hot rolled steel strip, the other steps of the inventive process may be substantially the same as those practiced in the prior art, and may be varied in accordance with prior art techniques. However, the primary object of the invention is to avoid the use of resulfurized steel during the hot rolling and primary reduction operations and, therefore, conventional nonresulfurized steel is the required starting material if the advantages of this invention are to be fully realized. Accordingly, the preferred embodirnent of this invention requires the casting and hot rolling of a conventional low-sulfur steel. The typical composition for such a steel would be carbon 0.05 to 0.12%, manganese 0.30 to 0. 60%, phosphorus 0.015% max.), sulfur 0.014 to 0.025%, the balance being iron, with other minor impurities in the usual residual amounts.

As noted above, the process of this invention is commenced in accordance with prior art practices. Hence, the nonresulfurized steel, as described above, is cast into ingot form and hot rolled in accordance with prior art practice to conventional hot band gage, i.e., about 0.08 inch. The extent of hot reduction is not critical. Thereafter, the hot rolled steel is cleaned to remove all surface scale and then cold rolled by prior art techniques, as previously described, to about 0.017 or 0.010. Although other gages may be produced by our process, these gages mentioned are the most common for container applications.

The crux of this invention resides in the treatment following the final cold roll whereby the steel sheet is immer'sed into an aqueous solution of from to 1.0 M of ferrous sulfate, FeSO and from 0.01 to 1.0 M sulfuric acid, H 80 and then withdrawn allowing the steel surfaces to dry. This will allow a small sulfate residue to adhere to the steel surfaces. Thereafter the steel is annealed in a reducing atmosphere containing hydrogen at a temperature of from 1100 to 1400" F. This anneal in a hydrogen containing atmospheres causes the sulfate residue on the steel surfaces to be reduced to ferrous sulfide which is firmly bonded to the sheet surfaces, providing a surface sulfur concentration of about 0.03%. Hence the steel strip, having its surface impregnated with sulfur, will .be resistant to the corrosive effects of carbonated beverages as would. be a conventional resulfurized steel. Yet, since high levels of sulfur are not present during the preceding rolling operations, the strip can be readily rolled without the attendant problems associated with rolling a resulfurized steel.

The dip solution, in its broadest sense, need be one which merely contains a small amount of sulfate ions, sufiicient to yield at least about 0.03% sulfur on the strip surfaces. Hence, other sources of sulfate ions can be used. Indeed, chromic sulfate, Cr (SO and stannous sulfate, SnSO have been used effectively in place of ferrous sulfate although MnSO and NiSO, proved to be inferior. In order to avoid contamination and minimize cost, it is of course most practical to utilize ferrous sulfate and sulfuric acid, and the practical limits thereof, as noted above are from 0 to 1.0 M ferrous sulfate and 0.01 to 1.0 M sulphuric acid. In view of the minimum limit of zero for ferrous sulfate, it is apparent that sulfuric acid can be used alone. Indeed, we have had successful trials using only sulfuric acid, but only when annealing temperatures exceeding 1200 F. In using sulfuric acid alone, however, the above minimum concentration of 0.01 M is a bit too low,- and should be at least about.0.02 M sulfuric acid. However, when ferrous sulfate is present in concentrations of at least 0.01 M, then the minimum concentration for sulfuric acid may be as low as 0.01 M. The use of ferrous sulfate alone, however, must be avoided as it may .be. partially converted to the sparingly soluble brownish colored, basic ferric sulfate in the presence of air. This slow oxidation process is minimized by acidifying the solution with sulfuric acid which lowers the solubility of oxygen in the solution.

Accordingly, a minimum dip solution concentration of about 0.01 M each of ferrous sulfate and sulfuric acid, or 0.02 M of sulfuric acid alone, is normally sufiicient to provide a strip having a surface sulfur concentration in excess of 0.03%. Although conventional resulfurized steel has a sulfur content limited to the close vicinity of 0.03%, surface sulfur contents in accordance with this invention may substantially exceed 0.03%, up to about 0.3%, and even more, without harmful effect. Therefore, dip solution concentrations may be as high as 1.0 M each of fer rous sulfate and sulfuric acid with equally acceptable results. Although concentrations of both ferrous sulfate and sulfuric acid can even exceed these values with acceptable results, such excessive concentrations should be avoided in order to minimize the risk of complications. Specifically, higher concentrations of sulfuric acid could result in excessive attack on the steel strip in the event of line slowdown or stoppage. On the other hand, excessive concentrations of ferrous sulfate can cause sludging by insoluble salts if the solubility limit is approached. It should also be noted that the surface sulfur concentration on the finished strip is not only a function of sulfate concentration in the dip solution, but to some extent, the line speed as well. That is, faster line speeds will drag out more solution on the strip surfaces causing a higher surface sulfur concentration. As a practical balance of all considerations, therefore, we prefer to use concentrations of 0.02 to 0.05 M for each ferrous sulfate and sulfuric acid.

For optimum results, we further prefer to clean the steel strip prior to the dip treatment. For example, we use a conventional electrolytic-alkaline cleaning followed by a water rinse. In addition, we prefer to maintain the dip solution at a temperature of about 180 F. to assure that the strip is readily wetted by the dip solution. Nevertheless, equally good Wetting can be achieved at room temperature if the steel surfaces are indeed clean. Since all that is necessary during the dip is that the steel surfaces be wetted by the solution it is obvious that holding times therein can be minimized to, say, one or two seconds. Still, longer holding times would not be detrimental if the sulfuric acid concentration is not sufiicient to attack the strip. After the dip, the strip should be dried before it is annealed. Before drying, however, it is preferable to pass the strip between grooved rolls to provide a uniform solution coating on the surfaces. To expedite drying for a continuous operation, heaters or warm air circulation can be used.

As in prior art practices, the annealing step of this inventive process is preferably performed in a continuous annealing furnace, although open-coil annealing may be utilized. In order to reduce the sulfate residue on the strip surface, the furnace atmosphere must be a reducing atmosphere containing hydrogen. The surface reaction is believed to be FeSO +4H FeS+4H O. Althrough a furnace atmosphere of hydrogen would work, we have found that the commercial HNX atmospheres, which usually contain from 2 to 10% hydrogen with a balance of nitrogen, are most suitable and preferred. Concentrations as low as 0.1% hydrogen are acceptable but require an extended holding time during the anneal. On the other hand, concentrations above 10% hydrogen are more than sufiicient. Minor amounts of impurity gases may 'be present without harmful effect.

As noted above, the annealing temperature should be within the range 1110 to 1400 F. As further noted, however, in the event ferrous sulfate is not included in the dip solution, then the annealing temperature must at least exceed 1200 F. Although the preferred annealing holding time is about 30'seconds, any period from about 15 seconds to 60' seconds or more may be used. In a box anneal, the holding time should be from about /2 to about 4 hours. At temperatures of 1200 F. and higher, the sulfate residue on the strip surface is rather quickly reduced to ferrous sulfide in the presence of hydrogen. Therefore, the annealing parameters are substantially the same as those used by the prior art as necessary to relieve the cold rolling stresses. In function, however, the anneal in this inventive process distinguishes over prior art anneals in that it not only serves to relieve the cold'rolling stresses, but further serves to reduce the sulfate residue to the desired ferrous sulfide. The presence of the sulfate residue on the steel surface does not in any way affect the-annealing process detrimentally, and in laboratory pilot line tests, there has been some evidence that the heat transfer characteristics of the strip are actually improved.

Since the sulfurizing treatment of this process provides well bonded ferrous sulfide in the strip, the strip can be further cold rolled without ill effect. Therefore, in accordance with some prior art processes, the strip produced by this process may be temper-rolled, if so desired, after the anneal.

EXAMPLES In one series of bench-scale tests, a low-carbon steel having the following composition: 0.076% carbon, 0.45% manganese, 0.005% phosphorus, 0.006% sulfur, 0.006%

silicon, 0.030% copper, 0.021% nickel, 0.005% chromium, 0.006% molybdenum and 0.004% aluminum, was cold-rolled in accordance with prior art practices as described above, dipped into various sulfate solutions at 180 F. and annealed at 1200 F. for either 30 seconds or 9.5 minutes in an HNX atmosphere containing 6% H Thereafter, the samples were tested for resistance to corrosion by carbonated beverages using the blue-dye-test (BDT). The samples were then subsequently cold rolled again in accordance with conventional double cold reduction practices, i.e., reduced from 0.008 to 0.005 inch, and again tested by the BDT. The BDT is a test developed by the American Can Company. for predicting base metal performance in carbonated beverages. The test is described in Corrosion Resistant Tin Plate for Carbonated Beverage Containers, by M. P. Mittelman, J. F. Collins and J. A. Lawson, Reg. Techn. Meeting Am. Iron Steel Inst, 1965, pp. 279-297. The test is based on a weight loss after 96 hours immersion of specimens of the metal (surface area, two sides, of 8.0 cm?) in a test medium made of phosphoric acid, deionized water, and FD&C (Federal Drug and Cosmetics approved) blue dye No. 1. Weight loss in milligrams is multiplied by 1.25 for conversion to microamperes per square centimeter a/cmF). The lower the BDT value, the better the product. BDT values below 60 a/cm. are considered acceptable. The dip solutions, annealing conditions and results of these bench-scale tests are shown in Table I below.

TABLE I Average BDT, aJemJ 16 min., 1,200 F. 9% min., 1,200" F. Dip treatment 1 Cold- C 1d- Cone. Cone. Reduced Reduced H 804, F6504, As alter As aiter M M annealed annealing 1 annealed annealing i No dip treatmen 66 146 45 120 0. 1 0. 1 34 23 20 19 0. 1 1. 0 20 17 15 14 1.0 0.1 16 18 14 12 1.0 1.0 17 18 18 14 0. 1 a 0. 1 18 21 14 15 0.1 4 0. 1 28 56 18 18 0. 1 0. 1 38 139 23 40 0. 1 0. 1 42 1 Solutions used at 180 F.

2 Cold reduced from 0.008 to 0.005 inch thickness. C1'2(S04):.

Nisot.

'1 Samples not prepared under these conditions.

In another series of bench-scale tests, the same steel identically processed was dipped into varying solutions of sulfuric acid and ferrous sulfate and annealed for 30 seconds at 1100, 1200 and 1300 F. in an HNX atmosphere containing 6% H Portions of the annealed samples were then tested by the BDT, while other portions of the annealed samples were further cold reduced from 0.008 to 0.005 inch and were then tested by the BDT. The results are shown in Table H below.

TABLE II Another series of tests were performed on commercial equipment using three coils of black plate produced in accordance with prior art practices noted above. The coils were dipped in an aqueous solution of 0.1 M FeSO -0.1 M H SO at F. at a line speed of f.p.m. The strip was then passed through grooved rolls to provide a uniform film thereon, then through a drying furnace at about 300 F. to dry the solution on the strip. Thereafter two coils were continuously annealed and one was box annealed. The continuous anneals (CA), at a line speed of 200 f.p.rn., provided a hold of from one to three minutes at 1300 to 1400 -F. in an atmosphere of from 7 to 12% H balance N The box anneal (BA) was for four hours in the range 1180 1220 F. with hydrogen in the incoming HNX atmosphere ranging from 4.95.8%. The annealed coils were then subjected to a second cold reduction and then tested to determine surface sulfur contents and BDT values. The results are shown in Table III below.

TABLE III Surface Sulfur Analyses and Blue-Dye Test Results for Black Plate Sampled After Second Cold Reduction Surface sulfur (Xray method) content, percent Blue-dye test Coil Type of Coil Sheet Top Bottom value, No. annealing position position surface suriace aJcrn E 0. 13 0. 21 Head C 0. 21 0.15 24 E 0. 21 0. 18 E 0. 18 0. 14 30 310623.. CA Center. 0 0. 21 0.16 27 E 0. 21 0. 15 E 0. 18 0. 12 Tail...-. C 0. 21 0. 13 25 E 0. 18 0. 21 E 0. 21 0. 14 Head... 0 0. 21 021 26 E 0. 21 0. 21 E 0. 21 0. 13 36 310625. CA Center 0 0. 21 0. 21 35 E 0. 21 0. 21 E 0. 20 0.11 Tall..... 0 0. 21 0. 21 26 E 0. 17 0. 21 E 0. 10 0.09 Head... 0 0.07 0. 08 21 E 0. 04 0. 06 E 0. 13 0. 10 17 310627- BA Center 0 0.05 0.05 20 E 0. 02 0.02 E 0. 04 0.04 Tail.-. C 0. 04 0. 04 25 E 0. 02 0. 02

1 Not tested.

Several other commercial-scale tests were conducted, some on metal which has been continuous cast through a commercial slab caster. In all these tests, equally good results were achieved.

We claim:

1. In a process for producing coated sheet steel stock for use in making food containers wherein a low-carbon steel is processed through steps which include hot rolling, cold rolling, annealing to relieve cold rolling stresses, and coating with a corrosion resistant material, the improvement comprising after cold rolling, immersing the steel into an aqueous solution of from 0.01 to 1.0 M

Bench Scale Experiments: Influence oi Dip Treatment in Aqueous Solutions at FeSO and/or H 804 Prior to Annealing in HNX on Blue Dye Test Values Average blue dye test values, naJcm.

% min., 1,100 F. /6 mln., 1,200 F. miu., 1,300 F.

Dip treatment 1 Cold- Cold- Cold- Cone. Cone. Reduced Reduced Reduced H2804, FeSOa, after after As aiter M M annealed annealing 1 annealed annealing 2 annealed annealing N0 dip treatment 169 149 168 144 162 0. 1 0 33 103 23 47 18 21 0. 1 0. 1 15 24 15 24 12 15 0. 1 0. 5 18 34 16 18 15 16 0. 1 1. 0 21 19 22 23 22 17 1. 0 0 16 23 16 17 17 14 1. 0 0. l 20 27 19 23 19 18 1. 0 1. 0 21 28 26 23 26 15 1 Solutions used at F. 1 Cold reduced from 0.008 to 0.005 inch thickness.

sulfuric acid and from 0.01 to 1.0 M of a metal sulfate selected from the group consisting of ferrous sulfate, chromic sulfate and stannous sulfate; withdrawing the steel from the solution and allowing the steel to dry leaving a sulfate residue thereon; and annealing the steel for at least 15 seconds at a temperature from 1l00-1400 F. in a reducing atmosphere containing at least 0.1% hydrogen to reduce said sulfate residue to ferrous sulfide surface coating to improve the steels resistance to the corrosive effects of carbonated beverages.

2. The method of claim 1 in which said solution contains from 0.02 to 0.05 M each of ferrous sulfate and sulfuric acid.

3. The method of claim 31 in which the reducing atmosphere contains from 2 to 10% hydrogen with a balance essentially of nitrogen.

4. The method of claim 1 in which the steel is further cold rolled following annealing.

5. The method of claim 1 in which the steel is further temper rolled following annealing.

6. The method of claim 1 in which the aqueous solution contains no metal sulfate and from 0.02 to 1.0 M sulfuric acid.

" 7. The method of claim 1 in which the anneal is a continuous anneal having a hold period of 15 to 60 seconds.

8. The method of claim 1 in which the anneal is a box anneal having a hold period of /2 to 4 hours.

References Cited UNITED STATES PATENTS 3,139,359 6/1964 Morgan l4812.1 3,653,990 4/ 1972 Hudson et al.

WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 148-14 

